Tertiary amine-oxides are known as alternative solvents for cellulose. From U.S. Pat. No. 2,179,181 is known for example that tertiary amine-oxides possess the property of dissolving cellulose without having to be converted into derivatives and that cellulose mouldings such as fibres can be made from these solutions by precipitation. In U.S. Pat. Nos. 3,447,939, 3,447,956 and 3,508,941 further processes for the preparation of cellulose solutions are described whereby the solvents used are preferably cyclic amine-oxides. In all these processes cellulose is physically dissolved at elevated temperatures.
If the solution is prepared in a twin-screw extruder or in a stirred vessel, the cellulose must be subjected to a preactivation procedure to ensure that the solution process is adequately fast (see "Das Papier", Vol 12, pp 784-788). As a preactivation procedure, the formation and regeneration of alkali-cellulose or a hydrothermal cellulose treatment is proposed.
Also according to DD-A 225 573, the cellulose is preactivated before preparing the solution, which is also carried out in an extruder. In the cited DD-A, the process starts from a cellulose suspension containing NMMO with a low consistency maximum of 2.5 wt % cellulose. This cellulose suspension is homogenised in a stirred vessel. Then by centrifuging or by pressing, the consistency is raised to 12.5 wt %, dried to a water content of 10-15 wt % (based on NMMO) and converted to a clear solution at temperatures between 75.degree. and 120.degree. C. in an extruder with a degassing zone.
The process according to the cited DD-A 226 573 suffers the disadvantage that after homogenisation the consistency must be increased from 2.5 wt % to 12.5 wt % before the actual preparation of the solution can commence. This necessitates a separate additional operating step. Furthermore, during the centrifuging or pressing procedures, not only water but also NMMO is removed which is undesirable.
The process of the Applicant described in EP-A 0 356 419 starts from cellulose, among other materials, which is milled in the dry state, The use of a dry milled cellulose allows it to be stirred into the aqueous NMMO solution in such large amounts that the process step for increasing the consistency becomes superfluous. This starting material, namely the dry milled cellulose, can however contain horny fragments which arise from local overheating during the milling process and these have an adverse effect on the solubility in aqueous NMMO. Dry milling to give fine particles also damages the cellulose structure. In this respect, dry milling to give larger particles would really be better but larger particles must preferably be excluded from the solution because otherwise the cellulose just becomes swollen at the surface and becomes difficult to dissolve, which leads to the formation of gel bodies and makes the cellulose solution difficult to filter.
The invention has as its object to simplify and improve the preparation of solutions of cellulose in tertiary amine-oxides.